tKENTON LIMESTONfi. 161 



208. 9. MODiOLOPSIS AVICULOIDES (n. «;jO' 

 PL. XXXVI. Figs. la,b. 



Obliquely ovate, ventricose ; cardinal line very oblique ; umbones prominent, with an 

 obtusely subangular ridge extending to the posterior ventral margin ; a slight parallel 

 depression just forward of the umbones ; posterior side somewhat extended and compressed, 

 having a winglike appearance ; anterior extremity pointed, and projecting slightly beyond 

 the umbones ; surface marked by fine concentric lines and a few prominent undulations. 



The specimen figured is somewhat imperfect, the compressed posterior margin being 

 broken off; and the anterior extremity has apparently suffered a little pressure. In its 

 general features it resembles other species of this genus, being closely allied to Modiolopsis 

 mytiloides. 



Fig. 1 a. A view of the left valve of this specimen, b. Dorsal view of the same. 



Position and locality. In the thin shaly layers which alternate with the more compact 

 ■trata in the central part of the Trenton limestone at Middleville. 



209. 10. MODIOLOPSIS? TRENTONENSIS. 



Pl. XXXV. Fig. 10. 



This figure is given by Mr. Conrad, as a species of CvPRicARDtTEs from the Trenton 

 limestone. It bears so many characters analogous to the larger species of Modiolopsis, that 

 I have referred it to that genus, giving the figure to call the attention of geologists to the 

 existence of such a species in the rock. 



210, 1. AVlCtlLA TRENTONENSIS. 



Pl. XXXVI. Figs. 2 a, b, e, d. 



Aticula trentonetuii. Conrad, Jour. Acad. Nat Sciences, Vol. viii. pag. 240, pi. 12, fig. 10. 

 — anformit. Id. lb. pag. 243, pi. 13, fig. 11. 



Obliquely obovate, gibbous, expanding towards the posterior extremity, which is some- 

 what obliquely truncated from the extremity of the hinge line, narrowed anteriorly, and 

 constricted just below the umbones ; anterior wing small, acute ; posterior wing narrow 

 and long, forming a kind of border to the shell 5 surface marked by strong radii and 

 concentric subimbricating ridges. 



An examination of several specimens has convinced me that the two species cited above 



are identical. In perfect specimens, the surface is marked by strong radii and less distinct 



concentric undulations. In worn or macerated specimens, the radii become obscure or 



obsolete, and the concentric elevated ridges become more distinct. The two valves are 



[ Palaeontology.] 21 



